SPECIFIC AIMS External and internal cues associated with the drug use environment often develop into persistent and powerful triggers for relapse to drug seeking. Many studies have reported that epigenetic enzymes, like histone deacetylases (HDACs), that alter chromatin structure and influence gene expression play an important role in addiction-relevant behaviors in animal models. In the previous grant period, we showed that the class IIa HDACs, HDAC5, plays important roles in the regulation of multiple cocaine-induced behaviors, including drug related learning and memory. The HDAC5 target gene, Npas4, emerged as an important, novel transcription factor required in the adult NAc for cocaine conditioned place preference and operant drug discrimination learning in the mouse intravenous self-administration. Our preliminary findings indicate that NPAS4 is induced in DARPP-32-positive medium spiny neurons (MSNs) and parvalbumin-positive fast spiking interneurons (PV- FSIs), but the relevant population(s) in which NPAS4 regulates drug behaviors remains unknown. Our long-term goal is to understand the role and regulation of NPAS4 in the development and/or persistence of drug addiction-related behaviors. Our central hypothesis is that NPAS4 induction within one or more subpopulations of activated NAc neurons promotes glutamatergic synaptic plasticity events that underlie long-lasting drug context/cue learning and memory. We will employ a series of multi-disciplinary, hypothesis testing experiments and cutting-edge molecular genetic approaches that will interrogate the in vivo cell type- specific role and regulation of NPAS4 and NPAS4-expressing cells in the NAc for drug addiction-related learning and memory. We will test and refine our central hypothesis with the following specific aims: Specific Aim 1: Determine the cell-type specific roles for NPAS4 during acquisition and extinction of drug self-administration. In this aim, we will take a cell-type specific loss- and gain-of-function approaches to study the function of NPAS4 during operant discrimination and extinction learning in the mouse drug self- administration assay. Specific Aim 2: Determine whether NPAS4-positive NAc neurons define a functional ?engram? population linking external cues to drug reward and seeking. Using our newly developed Npas4 enhancer-driven, tamoxifen-dependent cre virus, we will use a chemogenetic approach to manipulate the activity of the NPAS4-positive population and test its role in durg taking and seeking behaviors. Specific Aim 3: Determine the cellular function of NPAS4 in the NAc during drug SA. We will use cell- type specific loss- and gain-of-function strategies to evaluate the effects of NPAS4 on excitatory ad inhibitory synapse plasticity in mice self-administering cocaine or heroin.